issue #475: Properly set cert and key variables when starting nginx proxy

When specifying a custom SSL certificate and key, Kimchi did not start
up due the error:

Traceback (most recent call last):
File "/usr/bin/kimchid", line 95, in sys.exit(main(sys.argv[1:]))
File "/usr/bin/kimchid", line 92, in main kimchi.server.main(options)
File "/usr/lib/python2.6/site-packages/kimchi/server.py", line 183, in main
srv = Server(options)
File "/usr/lib/python2.6/site-packages/kimchi/server.py", line 62, in init
start_proxy(options)
File "/usr/lib/python2.6/site-packages/kimchi/proxy.py", line 89, in start_proxy
_create_proxy_config(options)
File "/usr/lib/python2.6/site-packages/kimchi/proxy.py", line 78, in _create_proxy_config
cert_pem=cert, cert_key=key,
UnboundLocalError: local variable 'cert' referenced before assignment

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>